Polyhydroxyalkanoates (PHAs) are polyesters accumulated by many prokaryotic organisms as intracellular carbon and energy reserve materials under limited nutrients, such as nitrogen or phosphorous. PHA polymers demonstrate thermoplastic or elastic properties depending upon their particular composition. These materials have attracted interest because of their potential use as biodegradable alternatives to petroleum-based synthetic plastics such as polypropylene and polyethylene. Efforts have been made to produce PHAs by microbial fermentation on glucose and organic acids. The production of biodegradable thermoplastics from organic wastes can provide multiple benefits to the environment, and contribute to sustainable development.
Organic wastes are usually complex in nature, and cannot be directly utilized by PHA-producing microbes such as Ralstonia eutropha, a representative bacterium for PHA synthesis. Hydrolysis and acidogenesis is the first step in converting the wastes to short-chain fatty acids such as acetic, propionic and butyric acids, that can be further utilized by PHA-producing bacteria. At present, there are technological difficulties in coupling the waste acidogenesis and PHA synthesis steps. In essence, both acid-producing and PHA-producing cells can be cultivated in a mixed culture, and the acids released by the former are directly utilized by the latter. The PHA content of the solid mass, however, is not high enough (10–30% by wt.) for effective polymer recovery, due to the considerable amount of non-biodegradable matter in the waste, as well as the biomass of non-PHA producing microbes. Furthermore, a high acid concentration due to the high microbial activity of acidogenic microbes would inhibit the microbial activity of PHA-producing cells.
An improved system could contain two separate bioreactors, in order to satisfy the different physiologies and metabolic activities of the two types of microbes; one for acidogenesis of organic wastes, and a second for an enriched culture of PHA-producing strains, such as R. eutropha. The fermentative acids should preferably be transferred from the first reactor to the second reactor without causing a solid mixing between the two reactors, so that an enriched culture of R. eutropha with a high PHA content can be maintained in the second reactor. Furthermore, the acid accumulation and toxicity in the PHA-producing reactor would also be under control.